you mean besides him ignoreing the fact that renekton stuns for 1.5sec with his combo, so you have actually less effective time to fight back vs his combo than vs fizz, or that renekton is manaless while fizz does blow alot mana when going for the full combo, or ignoreing that renekton has lots of waveclear while fizz has non(top fizz will not max E, and even if he does, it's not that great for waveclear without ap)
Or that unlike fizz, renekton has sustain and more base tankiness...
The main issues with fizz is that his lvl 1 is pretty absurd, haveing both sustained dps from the dot and burst for shortish trades from the W for low mana, and mostly that he's just really save, to a point where he often goes ignite instead of flash which gives him the edge in close matchups 1v1.
Yet his overall not as high damage/tankiness puts him somehwat into place. He's not nearly as broken as people make him out to be(though certainly strong), but he certainly is furstraiting to play against, mostly because he's really hard to shut down if he starts snowballing.